Increasing the power efficiency of stationary land-based industrial gas turbines will require future turbine inlet temperatures to increase to >1400° C. Cooling air will simultaneously need to be controlled to avoid an increase in NOx emissions. Similarly the concept of ultra low or “zero” emission power generation has focused on an oxy-fuel combustion process in which nitrogen is removed in the combustion air, and replaced with steam, thereby preventing the formation of nitrogen oxides. The exhaust stream of the oxy-fuel process is separated into concentrated CO2 and water. Alternately, hydrogen-fueled combustion turbine systems are conceptually based on a complex cycle composed of a closed Brayton cycle and a Rankine cycle. Hydrogen and oxygen are supplied as the fuel and oxidant respectively to a compressor, and burned in steam. The pressurized steam feed enters a high temperature turbine at 1700° C.
As increased efficiencies are sought in electric power generation, turbine-based power generation systems are increasingly required to operate under extreme conditions. The standard operating environment within a turbine is typically thermally and chemically hostile to materials used to form turbines and turbine components. As higher operating temperatures for gas turbine engines are continuously sought after in order to increase turbine efficiencies, which also increases the thermal and mechanical stresses placed on turbine materials. Therefore, as operating stresses increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack, and as a result may not retain adequate mechanical properties. For this reason, these components are often protected by a thermal barrier coating (TBC) system.
In order to successfully achieve long-term operation, higher operating temperature industrial land-based gas turbines will require internal blade cooling, potential utilization of advanced high temperature superalloys, and the incorporation of stable thermal barrier coating (TBC) systems. The following sections provide a brief overview of the current technology base.
Thermal Barrier Coating Systems
Industrial gas turbine engine components must withstand extreme temperatures, thermal cycling, and stress conditions. Current TBC systems consist of four layers, each consisting of different materials with specific properties and functions. These include the superalloy substrate, the bond coat, the thermally grown oxide layer (TGO), and the ceramic top coat.
Superalloy Materials
A superalloy or high performance metallic alloy is an alloy with superior mechanical strength, good surface stability, corrosion resistance, and is a material that can withstand high temperatures without oxidizing or losing mechanical properties. Creep and oxidation resistance are the prime design criteria for use of these materials. Superalloys are iron-, cobalt-, or nickel-based, the latter being suited for aeroengine applications. Many other elements can be present within the superalloy matrix. These include chromium (Cr), molybdenum (Mo), tungsten (W), aluminum (Al), zirconium (Zr), niobium (Ni), rhenium (Re), carbon (C), or silicon (Si).
Superalloys can be used at high temperatures, often in excess of 0.7 of the absolute melting temperature of the alloy. Nickel-based single crystal (SC) superalloys exhibit superior high temperature mechanical strength (at temperature >85% of their melting points), and hot corrosion resistance compared to conventional alloys (M. Kamaraj, “Rafting in single crystal nickel-base superalloys—an overview”, Sahana, Vol. 28, Parts 1 &2, February/April 2003, pp. 115-128).
The essential solutes in nickel-based superalloys are aluminum and/or titanium, with a total concentration that is typically less than 10 atomic percent. This generates a two-phase equilibrium microstructure consisting of gamma (γ) and gamma prime (γ). The γ′ phase is largely responsible for elevated temperature strength of the material, and its resistance to creep deformation. The strength of most metals is known to decrease as operating temperature is increased. In contrast, nickel-based superalloys containing the intermetallic γ′ compound are relatively insensitive to temperature.
Additions to commercial nickel-based superalloys include aluminum (Al) and titanium (Ti), chromium (Cr) small quantities of yttrium (Y) boron (B) or zirconium (Zr), and carbide formers C, Cr, Mo, W, Nb, Ta, Ti, and Hf can also be present. Carbides tend to precipitate at grain boundaries and reduce the tendency for grain boundary sliding. Elements such as cobalt (Co), iron (Fe), chromium (Cr), niobium (Nb), tantalum (Ti), molybdenum (Mo), tungsten (W), vanadium (V), titanium (Ti), and aluminum (Al) are known to be solid-solution strengtheners, both in γ and γ′ phases.
Currently nickel-based SC superalloys are identified for the manufacture of critical components as turbine blades, vanes for aircraft, as well as land-based power generation applications. Microstructure, chemical composition, and high temperature mechanical properties are the major factors controlling the performance of SC superalloys.
Bond Coat—Diffusion Barrier Coatings & Overlayer Coatings
TBC systems typically include an environmentally-protective bond coat and a thermal-insulating topcoat, typically referred to as the TBC. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element) as discussed in U.S. Pat. No. 4,321,310 and U.S. Pat. No. 4,321,311, and oxidation-resistant diffusion coatings such as diffusion aluminides that contain nickel-aluminum (NiAl) intermetallics as discussed in U.S. Pat. No. 5,238,752.
A bond coat is typically applied to the external surface of the superalloy to facilitate growth of a resistant oxide layer. The bond coat is the transition layer onto which the TBC topcoat is applied and adheres. Alumina (α-Al2O3) or an alumina-former is typically selected as a bond coat matrix as alumina offers excellent oxidation protection and has a very low growth rate. The two most widely used types of bond coatings are alumina-based matrices (Al/Al2O3), and MCrAlY where M=Fe and/or Ni. The former is obtained by surface enrichment (i.e., diffusion; see discussion below), while the later is achieved by plasma spray or electron beam physical vapor deposition (EB PVD). With the concept of introducing a diffusion barrier onto the surface of superalloys, the invention of Pt-aluminide coatings resulted. Platinum is typically electroplated to thicknesses of 5-10 μm. In low activity/outward diffusion coatings, the alloying elements present in the substrate (i.e., Ni) will tend to diffuse into the coating layer, to an extent limited by their solubility. In high activity/inward diffusion coatings, elements in solution enter the compound layer being formed, or form precipitates during processing.
In contrast, overlay coatings as opposed to diffusion coatings provide more independence from the substrate alloy, and also have more flexibility in design as compositions can be modified depending on the degradation mechanisms. Typically an overlayer coating as MCrAlY exhibits a two-phase β+γ microstructure. The presence of the γ-phase increases the ductility of the coating, thereby improving thermal fatigue resistance. MCrAlY's when used in land-based gas turbine applications contain nickel or cobalt. Chromium (Cr) is present to provide hot corrosion resistance. The concentration of Cr added is limited with respect to the specific alloy composition, as well as its capability to form Cr-rich phases within the coating. Cobalt (Co) is incorporated into the overlayer coating to provide superior resistance to corrosion. Aluminum (Al) when added in concentrations of 10-12 wt %, enhances oxidation life. Higher concentrations of Al reduce the ductility of the substrate metal. Yttrium (Y) enhances adherence of the oxide layer. The primary role of Y is its reaction with S to prevent sulfur segregation to the oxide layer which is detrimental to scale or coating adhesion. Alternately, addition of silicon (Si) improves cyclic oxidation resistance, but reduces the melting point of the coating. Rhenium (Re) when incorporated into the bond coat overlayer improves isothermal or cyclic oxidation resistance, and thermal cyclic fatigue. Tantalum (Ta) increases oxidation resistance.
In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC has and maintains a low thermal conductivity throughout the life of the component, including during high temperature excursions. However, the thermal conductivities of TBC materials such as YSZ are known to increase over time when subjected to the operating environment of a gas turbine engine. As a result, TBC's for gas turbine engine components are often deposited to a greater thickness than would otherwise be necessary. Alternatively, internally cooled components such as blades and nozzles must be designed to have higher cooling flow. Both of these solutions are undesirable for reasons relating to cost, component life and engine efficiency. As a result, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBC's are employed to thermally insulate components intended for more demanding engine designs.
Ceramic Top Coat—Thermal Barrier Coating
Top Coat
TBC top coat materials are typically ceramic materials and particularly zirconia (ZrO2) that is partially or fully stabilized by yttria (Y2O3), magnesia (MgO), ceria (CeO2), calcia (CaO), scandia (Sc2O3) or other oxides. Binary yttria-stabilized zirconia (YSZ) is widely used as a TBC top coat material because of its high temperature capability, low thermal conductivity and erosion resistance in comparison to zirconia stabilized by other oxides, e.g., ceria-stabilized zirconia, which exhibits poorer erosion resistance as a result of being relatively soft. YSZ is also preferred as a result of the relative ease with which it can be deposited by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. In plasma spraying processes, the coating material is typically in the form of a powder that is melted by a plasma as it leaves a spray gun. As a result, a plasma-sprayed TBC is formed by a buildup of molten “splats” and has a microstructure characterized by irregular flattened grains and a degree of inhomogeneity and porosity. TBC's top coats employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.).
Current thermal barrier coating (TBC) technology not only limits heat transfer through the coating, but also protects engine components from oxidation and hot corrosion. A 1-200 μm thick TBC coating can reduce temperature from the external surface of the coating to the superalloy interface by up to 200° C. The TBC can reduce the need for blade cooling by about 36% while maintaining identical creep life of the substrate.
During operation, the TBCs are exposed to various thermal and mechanical loads as thermal cycling, high and low cycle fatigue, and high temperature erosion. Due to reliability issues, the thickness of the coatings is limited in most applications to 500 μm. Increasing the coating thickness increases the risk of coating failure and leads to reduced coating life. The failure mechanisms that cause TBC coating spallation differ from traditional thinner coatings. The major reason for traditional TBC failure and coating spallation in gas turbines is considered to typically be related to bond coat oxidation. When the thickness of the thermally grown oxide (TGO) exceeds a certain limit, it induces critical stresses within the coating, which leads to failure. The thermally grown oxide (TGO) is formed by the oxidation of the bond coat layer when temperatures exceed 700° C. leads to the formation of the 1-10 μm thick thermally grown oxide layer. The oxidation results from the ingress of oxygen through the interconnected porosity of the ceramic top coat layer, or alternately through oxygen transport via high oxygen ionic diffusivity within the ZrO2-based ceramic top coat layer. Different chemical components such as sulfur, titanium, and tantalum degrade adhesion between the bond coat and the thermally grown oxide layer, while the addition of silicon and hafnium promote adhesion between the two layers.
Thicker coatings have higher temperature gradients through the coating and thus have higher internal stresses. Although the CTE of the traditionally accepted, commercial, yttria stabilized zirconia (YSZ) coatings is close to that of the substrate material, the CTE difference between the substrate and coating induces stresses at high temperatures at the coating interface. The phase structure of YSZ is not stable above 1250° C. Also the stain tolerance of the coating can be lost rapidly by sintering if too high surface temperatures are allowed. At temperatures >1000° C., YSZ ages via shrinkage and microcrack formation. The addition of 5-15% Y to Zr stabilizes the high temperature crystalline form, thus avoiding phase transitions at the service temperature range. Cerium and/or in combination with yttria has been incorporated as a stabilizer within the zirconia matrix. As an alternate to YSZ, lanthanum-hexaaluminate has been shown to exhibit long-term stability to temperatures of 1400° C. Its composition favors platelet formation which prevents densification of the coating by post-sintering.
Application of Thermal Barrier Coatings on Superalloys
Various methods have been used for depositing ceramic coatings onto metal substrates. These include physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes. Thermal spraying is a process that consists of melting a consumable (powder or wire) and projecting it as a molten particle onto the substrate. Upon impact with the substrate, the molten particle flattens and solidifies. Adhesion is initially mechanical in nature, and after further being subjected to a diffusion heat treatment, adhesion to the underlying substrate alloy is increased. A sprayed coating will have voids and oxide particles. All thermal spraying processes are sight-on-line—that is only the parts that are directly in the line of the spray are coated. Adhesion of the coating is dependent on the cleanliness of the substrate surface, its area (high roughness is desired), and the velocity of the particles. Various thermal spraying techniques include: Flame spraying: An oxyacetylene flame (2700° C.) is used to melt and project the coating fed as a wire or powder. Plasma spraying: An ionized gas plasma (1600° C.) is used to melt and propel the coating (powder). High velocity oxyfuel (HVOF): O2 and H2 are used with a fuel gas as methane to project the coating fed material. Low pressure plasma spraying (LPPS): This is accomplished similar to plasma spraying but done in an inert at low pressure. Vacuum plasma spray (VPS): Similar to plasma spray but done in an inert environment or under vacuum.
The two most widely used techniques for deposition of TBC top coats onto superalloy substrates are air plasma spray (APS) and electron-beam physical vapor deposition (EB PVD), both of which produce distinctively unique microstructures
Impact of Water Vapor on Material Stability
The presence of water vapor in air has been shown to adversely affect the selective oxidation of aluminum in superalloys (i.e., α-Al2O3-formers: PWA 1484, MarM 247, CM 186, Rene N5), by causing more transient oxides as NiO to be formed. Some adverse effects of water vapor on selective oxidation of aluminum do occur even at temperatures as high as 1100° C. Water vapor also causes the α-Al2O3 scale to crack and spall, particularly where the interfacial toughness between the α-Al2O3 scale and superalloy substrate is low.
The major effect of water vapor on superalloys that are chromia-formers (i.e., IN 738; X-40) is enhanced vaporization of Cr2O3, particularly at temperatures >900° C., as well as the formation of transient oxides. These chromia-forming alloys are resistant to cyclic oxidation degradation at 700° C. with no substantial effects of water vapor. These alloys are degraded rather severely at 900° C. This occurs due to cracking and spalling of the Cr2O3 scales which is exacerbated in wet air due to the vaporization of Cr2O3 via the formation of hydrated chromium oxides (CrO2(OH)2).
Considering applications for superalloys in environments containing water vapor, the chromia-formers should not be used at 900° C. or above due to vaporization of Cr2O3. In the case of the alumina-forming superalloys, problems related to the development of the α-Al2O3 scales at low temperatures such as 700° C. needs to be further addressed.
Corrosion of TBC Systems
When EB PVD ceramic-coated superalloys were exposed to sulfates and sintering agents at temperatures of 1204° C. for 16 hrs, penetration of contaminants into the intercolumnar spaces of the coating was identified. In addition, a potentially damaging reaction zone between PVD external (top) layer and the contaminants was clearly identified. Even more dramatic effects were identified after 1000 hrs of isothermal exposure of an EB PVD ceramic-coated matrix to lower temperatures (i.e., 1038° C.).
For the surface-modified PVD ceramics, failure by spallation resulted in the area between the hot zone on the trailing edge of the sample and the flame impingement zone on the leading edge of the sample. Ceramic spallation was accompanied by severe pitting of the bond coat. Where a section was completely removed, the bond coat was significantly depleted of the β-phase near the exposed surface, as well as near the interface with the base alloy. Failure occurred primarily within the thermally grown oxide layer.
A bond coat layer or a combined bond/top coat layer that provides for improved oxidation and corrosion resistance at higher temperature than present systems as well as improved water vapor stability at high temperatures would permit more efficient operation of turbine systems at higher temperature.